I have one of each - I've had the 650s for a couple years, and got the 700s a few months ago. I share Chadeffects' view that the 650s are a little dark, but I prefer that to overly bright headphones. I got the 700s because I had heard repeatedly that the 800s were very bright, and the 700s were a "toned down" variation. My personal view is that Sennheiser hit the sweet spot with the 700s. Very comfortable, beautifully balanced, transparent sound. Great imaging. In my view, definitely worth what they cost.
For those that care, I am running them off a Schiit Valhalla headphone amp connected to an Audio Research LS-27 preamp.
For those that care, I am running them off a Schiit Valhalla headphone amp connected to an Audio Research LS-27 preamp.